1. Field of the Invention
The present invention relates to a reference voltage circuit with improved temperature characteristics.
2. Description of the Related Art
As illustrated in FIG. 5, a conventional reference voltage circuit includes an N-channel depletion transistor 501 and an N-channel depletion transistor 502.
The operation is described. When a power supply voltage is sufficiently high, the N-channel depletion transistor 501 operates in the saturation region and the N-channel depletion transistor 502 operates in the triode region (variable resistance region). The aspect ratio (W/L) of the N-channel depletion transistor 501 is represented by A501; the threshold thereof, Vtd; the aspect ratio of the N-channel depletion transistor 502, A502; and the threshold thereof, Vtd. A voltage V521 at an output terminal 521 is determined as follows.
                    [                  Ex          .                                          ⁢          1                ]                                                                      V          521                =                              (                          1              -                                                                                          A                      502                      2                                        +                                                                  A                        501                                            ·                                              A                        502                                                                                                              A                  502                                                      )                    ⁢                      V            td                                              (        1        )            
A temperature gradient of the voltage V521 is determined as follows.
                    [                  Ex          .                                          ⁢          2                ]                                                                                  ⅆ                          V              521                                            ⅆ            t                          =                              (                          1              -                                                                                          A                      502                      2                                        +                                                                  A                        501                                            ·                                              A                        502                                                                                                              A                  502                                                      )                    ⁢                                    ⅆ                              V                td                                                    ⅆ              t                                                          (        2        )            
As is apparent from Expressions (1) and (2), the conditional expressions of the absolute value of the output voltage V521 and the temperature gradient are determined only by the thresholds and the channel aspect ratios of the depletion transistors and include no terms affected by the mobility.
In general, the temperature gradient of the mobility is nonlinear. The temperature gradient of the threshold, on the other hand, is known to be regarded as linear at about −1 to −2 mV/° C. If the ratio of the aspect ratios of the N-channel depletion transistor 501 and the N-channel depletion transistor 502 is adjusted to 8:1 as a realistic value, the value of the output voltage V521 is |2×Vtd|, and the temperature gradient is given as −2 times the temperature gradient of the same threshold.
As described above, the mobility is not involved in the elements that determine the output voltage and the output characteristics, and hence the output voltage and the output characteristics are determined only by the thresholds of depletion transistors and the ratio accuracy in layout. Further, there are a small number of elements that have manufacturing fluctuations, and hence a stable output can be obtained (see, for example, Japanese Patent Application Laid-open No. 2007-24667 (FIG. 5)).
In the conventional technology, however, a constant gradient is present with respect to temperature, and hence there is a problem in that it is not suitable for a reference voltage circuit which is required to have flat temperature characteristics